CEL 亮了，code是P0420 Catalyst Efficiency Low - bank1,应该按啥步骤排查？

explanation:
The oxygen sensors monitor the Catalytic Converters ability to store oxygen.
Problem Cause:
1. Catalytic converter defective ( failure possibly due to #2,2,or4)
2. Engine misfire or running conditon
3. Large vacuum leak
4. Engine oil leakage into exhaust-valve guide seals, piston rings

车是1997年Ford Aspire. 1.3L, Automatic. 我应该先查啥？如果是catalyst converter 堵了，哪位给推荐一个catalyst converter cleaner. 谢谢